Mass spectrometry techniques, particularly those requiring the measurement of stable (non-radioactive) nuclide enrichment in biological samples, have become established tools in the study of human nutrition and metabolism. Such methods, though technically demanding, are uniquely suited to quantitative In vivo studies under various pathophysiologic states. The Core's initiative to combine whole body tracer approaches with tissue specific mobile PET imaging is a novel means to evaluate metabolic alterations and has significant applicability to obese and non-obese subjects. Likewise, the addition of a metabolomics and lipidomics component to the Core will further enhance the scope of nutritional and metabolic studies available to the NORC-H investigators. This Core, its Director and Co-Directors, as well as its senior technician will provide: 1. Assistance in the design of successful nutritional stable isotope, and PET experiments including tracer selection, isotope preparation for infusion, and consultation regarding institutional review board approval. 2. A metabolomics platform capable of biomarker and metabolite discovery, biomarker and metabolite identification and metabolite quantitation. 3. Efficient and equitable access to Core analytic instruments, accurate analyses of samples, and quality control. 4. Assistance in the development of new methods for analyses of compounds in biologic samples including blood, urine, tissues and expired air. 5. An opportunity for new investigators to learn about the application of stable isotope-tracer techniques, PET and metabolomics in metabolic/nutritional research.